In addition to a variety of other applications, nonwovens are very useful for creating filter media. Such nonwoven webs include meltblown fiber webs and spunbond fiber webs. By way of example, U.S. Pat. No. 5,460,007 to Midkiff et al. discloses the use of a nonwoven web for HVAC (heating, ventilating and air-conditioning) and other air filtration media. PCT Application No. US94/12699 (Publication No. WO95/13856) discloses high-loft multicomponent fiber webs suitable for use in a variety of air filtration applications. Additionally, U.S. Pat. No. 5,855,784 to Pike et al. discloses a variety of conjugate fiber nonwoven webs suitable for use as air and/or liquid filtration media. Further, multilayer laminates have likewise been used in a variety of filtration and/or filtration-like applications as discussed, for example, in U.S. Pat. No. 5,721,180 to Pike et al. and U.S. Pat. No. 4,041,203 to Brock et al.
Filter media fabricated from meltblown fiber webs tend to provide high filtration efficiency because of the fine fiber size and the conformability of meltblown fibers that causes the fibers to come together as a dense, fine-pored web. The resulting interfiber pore structures are highly suitable for mechanically trapping or screening fine particles. Additionally, the efficacy of various materials as filtration media, including meltblown fibers, can be improved when such is permanently, electrostatically treated, such as by applying a high-voltage electric field to the material. The resulting electret can be particularly suitable for the filtration of aerosols. A dielectric becomes an electret when the rate of decay of the field-induced polarization can be slowed down so much that a significant fraction of the polarization is preserved long after the polarizing field has been removed. Such electrets can be made by various methods, e.g. corona charging, triboelectric charging (friction) and so forth. By way of example, methods of treating various materials such as meltblown fibers in order to impart an electrostatic charge are described in U.S. Pat. No. 4,215,682 to Kubic et al., U.S. Pat. No. 4,375,718 to Wadsworth et al., U.S. Pat. No. 4,588,537 to Klaase et al. and U.S. Pat. No. 5,401,446 to Tsai et al, which are incorporated by reference. Improvements in creating electret materials are disclosed in U.S. Pat. No. 6,573,205 to Myers et al. and U.S. Pat. No. 6,893,990 to Myers et al., which are also incorporated here by reference.
Unfortunately, meltblown fiber webs typically do not have a physical integrity that is self-supporting and such are susceptible to delamination and abrasive forces during handling and converting. The structural integrity of such webs can be improved by increasing the basis weight or thickness of the web, and protection from abrasion and delamination can be achieved by incorporating the meltblown fibers inside a spunbonded portion of the filter media. However, increasing the basis weight or thickness exacerbates the pressure drop across the filter media. Furthermore, adding support structure or incorporating the meltblown fibers inside a spunbonded portion for protection from abrasive forces does not typically contribute to the filtration process and generally increases the production cost of the filter media. Additionally, the complexity of manufacturing a filter media from meltblown fiber webs has heretofore been further complicated by the time during which the meltblown fibers remain tacky or adherent. Attempting to wind or roll meltblown fibers while such remain adherent can lead to problems such as meltblown fibers sticking together as the material is wound onto a roll, which is sometimes referred to as blocking the roll of materials.
Accordingly, a need exists for a material that can be used to construct, for example, porous media that is resistant to delamination and abrasive forces during handling and converting. A need also exists for such a material that can be also be used to construct a filter media without the necessity of multiple structural support layers or unnecessary pressure drop across the media. A filter media that can also be wound or further processed relatively quickly after manufacture without concerns for blocking would be beneficial.